Far be it from me to only harp on Hitman: Absolution's less admirable qualities. The game is also frequently funny, and often very well-written. The overheard dialogue is often worth sticking around and listening to, and the crowd chatter is among the most convincing I've ever heard in a game. (The game itself is a lot of fun, too.)
I've cut together a few good examples of quality overheard dialogue (there are some slight location spoilers, but nothing huge), each of which demonstrate how much fun the writers and voice actors had with the material. I'm not sure what it takes to make a one-sided cell conversation sound so convincing (maybe you write the whole exchange, then leave half out?), but time and again, these writers pulled it off.